There are certainly times when it's acceptable to use a mirror while you work out, but for the most part, I recommend not relying on a mirror during training.
Without the visual feedback a mirror provides, you are forced to develop better kinesthetic awareness. Kinesthetic awareness is your ability to detect changes in your body's position without relying on your senses.
I believe that having at least decent kinesthetic awareness is something every person should strive to develop.
Having an understanding of how your body should be arranged, how it should move, and how it should interact with other objects in space is something that has carryover to training as well as everyday life.
When it comes to training, the better your kinesthetic sense, the stronger your mind to muscle connection will be, and you will be less likely to sustain an injury.
As extreme as this may sound, a person who has great spatial awareness, is less clumsy, practices sound body positions, and utilizes safe movements is better equipped for long-term survival.
Again, I am not saying that you should never use a mirror while you are working out.
I am only saying that you should train yourself to not depend on one every time you step into the gym.